1. Field of the Invention
This invention relates to sea floor template apparatus for use in the drilling of wellbores from offshore drilling structures, such as ships and platforms, and more particularly to wellhead supported subsea templates and methods of installing such templates.
2. Setting of the Invention
In offshore drilling of oil and gas wells it is advantageous to locate a plurality of wellbores close together and in a known orientation. As a result, a variety of subsea templates have been developed and used for fixing the orientation of wellbores, and for guiding the drill strings and production conduits and tools therein. Such templates include a plurality of laterally spaced guide slots which establish the horizontal location of wellbores to be drilled and are thereafter used to receive and guide the conduits and tools utilized. Certain of the known templates are placed directly on the sea floor and rely on their great mass to maintain themselves in position. A problem encountered in using these types of templates is that the templates have often become fully or partially submerged in mud or they have settled unevenly as a result of ocean floor erosion. The submerging or uneven settling of the templates has often resulted in inhibiting the passage of drill pipe, other conduits, and apparatus through the guide slots of the template and has required the carrying out of expensive remedial operations. Such sea floor supported templates are also subject to rotation after placement by action of strong currents which causes the orientation of the guide slots of the templates to be changed and other difficulties.
Other types of templates are supported by or are rigidly attached to pilings driven into the sea floor. While these templates do not encounter the problems of submerging, uneven settling, or rotation as the previously described templates, these pile-supported templates require the time-consuming and expensive pile driving procedure.
A subsea template which is supported above the sea floor by a single piling is disclosed in U.S. Pat. No. 3,618,661 issued on Nov. 9, 1971. Such template, compared to other piled templates, however, still requires the installation of a piling in the sea floor.
A more fundamental disadvantage to using templates that rest on the sea floor or are pile supported is that these templates are installed prior to drilling a wellbore. In other words, the drilling of a wellbore to produce oil and/or gas must be accomplished after the template is in place. If insufficient oil and/or gas are found, then no further wells will be drilled and the template is either recovered or abandoned. There is a need for a template which is installed after one wellbore has been drilled and the decision has already been made to drill additional wellbores adjacent thereto.
An example of a template that is connected to an existing wellhead is described in an article entitled "Offshore Construction--Drillship Sets Three-Well Template" which appears in the March 1978 issue of Ocean Industry Magazine at page 51. While such a template apparatus avoids the necessity of installing one or more pilings, because it rests on the sea floor, it is still subject to undesirable setting and rotation. Further, it requires that a diver attach the template by means of a gate or clamp to the existing wellhead. There is a need for a subsea template that engages to an existing wellhead and is supported above the sea floor. Further, there is a need for such a template that does not require the use of divers to install.